The 291st prohibition is that a witness is forbidden from saying anything extra regarding the case he is testifying about. Even if he is learned and knowledgeable, he may not act as a witness and a judge and legal authority. He must testify concerning what he saw and then remain silent, and the judges will use his testimony as they see fit. The witness is prohibited from saying anything in addition to his testimony. This law applies only to capital cases.
The source of this prohibition is G‑d's statement (exalted be He), "A single eyewitness may not speak against a person where the death penalty is involved." This prohibition is repeated in the verse, "He shall not be put to death through the testimony of one witness." This means that he shall not be put to death because of the instructions of the witness.
Our Sages said in tractate Sanhedrin, "The prohibition, 'A single eyewitness may not speak against a person' applies whether he speaks for acquittal or for conviction," and they give a reason, "Because it will appear that he is a biased witness." The prohibition against speaking out either for or against the defendant applies only in capital cases.